Fabrication of silicon-germanium fin structure having silicon-rich outer surface

ABSTRACT

A method includes forming an oxide layer on a silicon-germanium (SiGe) fin formed on a substrate. The first oxide layer comprises a mixture of a germanium oxide compound (GeO x ) and a silicon oxide compound (SiO x ). The first oxide layer is modified to create a Si-rich outer surface of the SiGe fin. A silicon nitride layer is deposited on the modified first oxide layer.

BACKGROUND

Semiconductor structures or devices may be embodied as field-effecttransistors (FEB), such as metal-oxide-semiconductor FETs (MOSFETs). Oneor more such MOSFETs comprise an integrated circuit (IC or chip). AFinFET is a nonplanar MOSFET that may be built on a silicon substrate,such as a silicon-on-insulator (SOI) substrate. FinFETs may provideimproved areal density and gate control of the conducting channel overplanar MOSFETs.

SUMMARY

Illustrative embodiments of the invention provide techniques forfabricating improved semiconductor structures. While illustrativeembodiments are well-suited to improve operation of FinFET structures,in particular, silicon-germanium FinFETs, alternative embodiments may beimplemented with other types of semiconductor structures.

For example, in one illustrative embodiment, a method for fabricating asemiconductor structure comprises forming a first oxide layer on asilicon-germanium (SiGe) fin formed on a substrate. The first oxidelayer comprises a mixture of a germanium oxide compound (GeO_(x)) and asilicon oxide compound (SiO_(x)). The first oxide layer is modified tocreate a Si-rich outer surface of the SiGe fin. A silicon nitride layeris deposited on the modified first oxide layer.

More particularly, in a further illustrative embodiment, the method forfabricating a semiconductor structure further comprises performing ashallow trench isolation (STI) process, with a silicon nitride layerprotecting the SiGe fin during the STI process. In one example,performing the STI process comprises forming a second oxide layer on thesilicon nitride layer and performing an annealing process on the secondoxide layer for oxide densification. An improved semiconductor structureis thereby formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional side view of a portion of a SiGeFinFET with which one or more embodiments of the invention will bedescribed.

FIG. 2A is a schematic cross-sectional side view of a portion of a SiGeFinFET at a first-intermediate fabrication stage, according to anembodiment of the invention.

FIG. 2B is a schematic cross-sectional side view of a portion of a SiGeFinFET at a second-intermediate fabrication stage, according to anembodiment of the invention.

FIG. 2C is a schematic cross-sectional side view of a portion of a SiGeFinFET at a third-intermediate fabrication stage, according to anembodiment of the invention.

FIG. 2D is a schematic cross-sectional side view of a portion of a SiGeFinFET at a fourth-intermediate fabrication stage, according to anembodiment of the invention.

FIG. 2E is a schematic cross-sectional side view of a portion of a SiGeFinFET at a fifth-intermediate fabrication stage, according to anembodiment of the invention.

FIG. 2F is a schematic cross-sectional side view of a portion of a SiGeFinFET at a sixth-intermediate fabrication stage, according to anembodiment of the invention.

DETAILED DESCRIPTION

In illustrative embodiments, techniques are provided for transistorfabrication for implementation in a semiconductor structure. Moreparticularly, illustrative embodiments provide techniques forfabricating silicon-germanium (SiGe) FinFETs. As will be explained inillustrative embodiments, such fabrication techniques advantageouslyreduce performance degradation by reducing, for example, Ge pile-up atthe periphery and interference traps.

Furthermore, it is to be understood that embodiments discussed hereinare not limited to the particular materials, features, and processingsteps shown and described herein. In particular, with respect toformation (fabricating or processing) steps, it is to be emphasized thatthe descriptions provided herein are not intended to encompass all ofthe steps that may be used to form a functional integrated circuitdevice. Rather, certain steps that are commonly used in forming suchdevices are purposefully not described herein for economy ofdescription.

Moreover, the same or similar reference numbers are used throughout thedrawings to denote the same or similar features, elements, layers,regions, or structures, and thus, a detailed explanation of the same orsimilar features, elements, layers, regions, or structures will not berepeated for each of the drawings. It is to be understood that the terms“about,” “approximately” or “substantially” as used herein with regardto thicknesses, widths, percentages, ranges, etc., are meant to denotebeing close or approximate to, but not exactly. For example, the term“about” or “substantially” as used herein implies that a small margin oferror is present such as, by way of example only, 1% or less than thestated amount. However, other percent margins of error may be realized.Also, in the figures, the illustrated scale of one layer, structure,and/or region relative to another layer, structure, and/or region is notnecessarily intended to represent actual scale.

FinFET fabrication methods have been developed to replace pure silicon(Si) fin structures, or fins, with silicon-germanium (SiGe) fins. Ascompared to forming the fins from pure Si, forming the fins from SiGereduces threshold voltage (Vt), thereby increasing the current thatflows through the channel. Additionally, SiGe fins provide for a highercarrier mobility than their pure Si fin counterparts.

FinFETs may be associated with a certain amount of off-state currentleakage, which is current leakage that remains after the FinFET isturned “off”. One component of the off-state leakage is punch-throughleakage. If the off-state current leakage is not accounted for, anon-negligible off-leakage current density may be realized just belowthe channel bottom region of the FinFET.

FIG. 1 illustrates a schematic cross-sectional side view of a portion ofa SiGe FinFET 100. As shown, SiGe FinFET 100 includes a silicon (Si)substrate 102. Formed on substrate 102 is fin 104. As shown, fin 104includes portion 104 a and portion 104 b. Portions 104 a and 104 b areeach comprised of SiGe. For example, and as shown in this illustrativeembodiment, portion 104 a is associated with a 40% SiGe doping profile,and portion 104 b is associated with a 25% SiGe doping profile. As isknown, a SiGe doping profile is the percentage of dopant in the SiGematerial (e.g., a fin portion that has a 40% SiGe doping profile meansthat the fin is comprised of a mixture of about 60% dopant and about 40%silicon-germanium).

Oxide layer 106 is formed on opposing sides of SiGe fin 104 and isfurther in contact with substrate 102. Oxide layer 106 is a passivationlayer formed by the Si and Ge of fin 104 coming in contact withatmospheric oxygen. Oxide layer 106 may comprise a mixture of a siliconoxide (SiO_(x)) compound and a germanium oxide (GeO_(x)) compound. Forexample, oxide layer 106 may comprise a mixture of silicon dioxide(SiO₂) and germanium dioxide (GeO₂).

As further shown, silicon nitride layer 108 is formed on oxide layer 106and in further contact with substrate 102. In one embodiment, siliconnitride layer 108 is comprised of silicon mononitride (SiN). A mainfunction of silicon nitride layer 108 is to provide protection for fin104 during one or more stages associated with a device isolationprocess. Device isolation may be used to prevent adjacent semiconductorcomponents (e.g., separate transistors) from interacting with each othervia the substrate. For example, device isolation may be used forpreventing electric current leakage between SiGe FinFET 100 and one ormore adjacent semiconductor devices. One such device isolation processis shallow trench isolation (STI). Accordingly, silicon nitride layer108 protects fin 104 from one or more adverse effects that may occur dueto one or more steps performed during a STI process.

A second oxide layer, oxide layer 110, is formed on silicon nitridelayer 108 and in further contact with substrate 102. Oxide layer 110 isan oxidized STI liner formed by chemical vapor deposition (CVD) duringthe above-mentioned STI process. CVD is a chemical process used toproduce conformal films and augment substrate surfaces. Generally, CVDis performed by exposing the substrate to one or more volatileprecursors, which react and/or decompose on the substrate to produce thedeposition. Any volatile by-products of the CVD process may be removedby gas flow through the reaction chamber used during the CVD process.

As further shown in FIG. 1, gate stack 112 is formed along fin 104. Gatestack 112 regulates the conductivity of SiGe FinFET 100 via voltageapplication to gate stack 112 (i.e., gate voltage). The applied gatevoltage imposes an electric field into SiGe FinFET 100, therebycontrolling the flow of charge carriers through the channel between asource terminal of fin 104 (not shown) and a drain terminal of fin 104(not shown). The density of the charge carriers affects the conductivitybetween the source and the drain.

It is realized herein that an unwanted Ge pile-up at the SiGe FinFETperiphery, denoted in FIG. 1 as Ge pile-up 114, is observed during thefinal stages of SiGe FinFET fabrication e.g., at the fin reveal process.This may be a root cause of high interface states resulting indegradation of SiGe FinFET device performance. The Ge pile-up at theSiGe FinFET periphery may come from the STI liner formation, which maybe carried out at higher temperatures with in-situ steam generation(ISSG). In one example, the STI liner formation is carried out at atemperature of 850° C. Such a temperature is generally too high for SiGeFinFET oxide liners, which may cause the Si to be selectively oxidizeddue to a lower Gibbs free energy as compared to Ge. This hightemperature oxidation may result in severe SiGe FinFET loss during theISSG process. Additionally, a large number of interference traps at thebottom of the SiGe FinFET and

STI liner may be a root cause of off-leakage current path andsubthreshold swing degradation in SiGe FinFETs.

FIGS. 2A-2F illustrate an exemplary process of fabricating asemiconductor structure that addresses one or more of the aforementionedproblems.

FIG. 2A illustrates a SiGe FinFET 200 at a first intermediatefabrication stage. For the purpose of clarity, several fabrication stepsleading up to the production of the SiGe FinFET 200 as illustrated inFIG. 2A are omitted. In other words, the SiGe FinFET 200 does notnecessarily start out in the form illustrated in FIG. 2A, but maydevelop into the illustrated structure over one or more well-knownprocessing steps which are not illustrated but are well-known to thoseof ordinary skill in the art. Also, note that the same reference numeral(200) is used to denote the semiconductor structure (SiGe FinFET)through the various intermediate fabrication stages illustrated in FIGS.2A through 2F. Note also that semiconductor structure 200 can also beconsidered to be a semiconductor device and/or an integrated circuit, orsome part thereof.

SiGe FinFET 200 comprises a substrate 202. In one embodiment, substrate202 is a silicon (Si) substrate. For example, substrate 202 may be asilicon-over-insulator (SOI) substrate. Fin 204 is formed on substrate202. As shown, fin 204 includes portion 204 a and portion 204 b. In oneembodiment, and as shown, portion 204 a is associated with a 50% SiGedoping profile, and portion 204 b is associated with a 25% SiGe dopingprofile. However, it is to be appreciated that the doping percentagesprovided herein are purely exemplary and are not to be consideredlimiting.

As shown in FIG. 2A, oxide layer 206 a is formed on fin 204, similar tooxide layer 106 discussed above in the context of FIG. 1. Oxide layer206 a is an oxide comprised of a mixture of a germanium oxide compound(GeO_(x)) and a silicon oxide compound (SiO_(x)). In one embodiment, thegermanium oxide compound is GeO 2 , and the silicon oxide compound isSiO 2 . Various oxidation methods that may be utilized in the formationof oxide layer 206 a in accordance with the embodiments described hereinare known in the art, such as thermal oxidation, ozone oxidation, plasmaoxidation and radical oxidation. For example, in one embodiment, oxidelayer 206 a may be formed utilizing an ozone-rich deionized watersolution (e.g., DIO₃). An exemplary zoom-in view 208 a of fin 204 andoxide layer 206 a is provided to further illustrate the formation ofoxide layer 206 a. Note that the chemical bonds toward the right side ofview 208 a represent the oxide layer 206 a, while the chemical bondstoward the left side of view 208 a represent fin 204.

FIG. 2B illustrates SiGe FinFET 200 at a second intermediate fabricationstage. In this stage, oxide layer 206 a from FIG. 2A is modified tocreate a Si-rich outer surface of fin 204. Specifically, oxide layer 206a is converted into oxide layer 206 b by removing GeO_(x) from oxidelayer 206 a leaving SiO_(x), as illustrated by the exemplary zoom-inview 208 b of fin 204 and oxide layer 206 b. In one embodiment, removingthe GeO_(x) from oxide layer 206 a comprises scavenging the GeO_(x) viaan annealing process. For example, the following reaction may occur withfin 204 and oxide layer 206 a: Ge+GeO₂→2GeO. This reaction results inthe diffusion of GeO from the oxide layer 206 a, i.e., GeOout-diffusion. The Ge atoms from the surface of fin 204 are consumedduring the modification of oxide layer 206 a to oxide layer 206 b viaGeO out-diffusion, thereby providing the Si-rich outer surface of fin204.

As illustratively used herein, the term “Si-rich outer surface” may bedefined as an outer surface depleted of Ge such that the outer surfaceis fully or at least predominantly comprised of Si. Note that thechemical bonds toward the right side of view 208 b represent the oxidelayer 206 b, while the chemical bonds toward the left side of view 208 brepresent fin 204. Thereby, as will be illustrated further in FIG. 2F,fin 204 is considered to have an outer surface that is Si-rich (i.e.,depleted of germanium, and fully or at least predominantly silicon). Inone embodiment, oxide layer 206 b has a thickness of about 10 Angstroms.

As is known in the art, an annealing process is a heat treatment processthat increases the ductility of a material by altering physical and/orchemical properties of the material. A general annealing process may becarried out in a temperature range, for example, from about 260° C. to760° C. In one embodiment, the annealing process employed herein iscarried out at a temperature range from about 600° C. to 750° C. Thehigh temperature utilized in the annealing process may result inoxidation of the surface of the material. To avoid unwanted oxidation,the annealing process may be carried out in a controlled atmosphere. Inone embodiment, the annealing process may be carried out in a controlledatmosphere comprising an endothermic gas (i.e., a controlled atmospherecomprising one or more of carbon monoxide (CO), hydrogen gas (H₂) andnitrogen gas (N₂)). In another embodiment, the annealing process may becarried out in a dissociated ammonia atmosphere, which is a controlledatmosphere comprising a forming gas (i.e., a controlled atmospherecomprising a mixture of H₂ and N₂).

FIG. 2C illustrates SiGe FinFET 200 at a third intermediate fabricationstage. In this stage, silicon nitride layer 210 is deposited on the topof oxide layer 206 b and in contact with substrate 202. For example,silicon nitride layer 210 may be comprised of silicon mononitride (SiN).In one embodiment, silicon nitride layer 210 has a thickness of about 45Angstroms.

FIG. 2D illustrates SiGe FinFET 200 at a fourth intermediate fabricationstage. In this stage, oxide layer 212 a is formed on the top of siliconnitride layer 210 and in contact with substrate 202. In one embodiment,oxide layer 212 functions as an STI liner, as discussed above withreference to FIG. 1. The formation of oxide layer 212 a may take placein two stages. First, oxide layer 212 a is deposited on the top ofsilicon nitride layer 210. In one embodiment, oxide layer 212 a isdeposited via a chemical vapor deposition (CVD) process, similar to theprocess discussed above in FIG. 1. Second, after oxide layer 212 a isdeposited, an annealing process may occur for oxide densificationpurposes. One function of silicon nitride layer 210 is to protect fin204 during this annealing process.

FIG. 2E illustrates SiGe FinFET 200 at a fifth intermediate fabricationstage. In this stage, oxide layer 212 a from FIG. 2D is converted intooxide layer 212 b by removing oxide layer 212 a corresponding to firstportion 204 a of fin 204. In one embodiment, the removal of oxide layer212 a to form oxide layer 212 b is performed via a chemical-mechanicalplanarization (CMP) process. Accordingly, oxide layer 212 b is arecessed version of oxide layer 212 a to expose the portion of siliconnitride layer 210 corresponding to portion 204 a of fin 204.

FIG. 2F illustrates SiGe FinFET 200 at a sixth intermediate fabricationstage. In this stage, portion 204 a of fin 204 is revealed.Specifically, portion 204 a of fin 204 is revealed by stripping theportion of silicon nitride layer 210 and the portion of oxide layer 206b corresponding to portion 204 a of fin 204. An exemplary zoom-in view214 of portion 204 a of fin 204 is provided to further illustrate theresult of the stages described above in FIGS. 2A-2F. As shown, the outerlayer of portion 204 a of fin 204 not interfacing with portion 204 b offin 204 is converted to a Si-rich layer, while the inner layers ofportion 204 a are comprised of SiGe Accordingly, a uniform profilethroughout fin 204 is achieved.

Advantageously, illustrative embodiments reduce off-leakage currents inSiGe FinFETs. Having a Si-rich surface in the SiGe channel, as providedherein, is generally preferable to a surface having a high Geconcentration in order to reduce interference trap density.Additionally, fabricating a SiGe FinFET having a Si-rich surface withoutrequiring conventional epitaxial deposition of Si provides advantageswith respect to reliable gate stack formation. For example, theconventional epitaxial deposition of Si cannot be effectively controlledless than 1 nm on the fin structure of the SiGe FinFET, which affectsdeposition uniformity. The embodiments described herein provide forincreased deposition uniformity over such conventional depositionmethods.

It is to be understood that the methods discussed herein for fabricatingsemiconductor structures can be incorporated within semiconductorprocessing flows for fabricating other types of semiconductor devicesand integrated circuits with various analog and digital circuitry ormixed-signal circuitry. In particular, integrated circuit dies can befabricated with various devices such as transistors, diodes, capacitors,inductors, etc. An integrated circuit in accordance with embodiments canbe employed in applications, hardware, and/or electronic systems.Suitable hardware and systems for implementing the invention mayinclude, but are not limited to, personal computers, communicationnetworks, electronic commerce systems, portable communications devices(e.g., cell phones), solid-state media storage devices, functionalcircuitry, etc. Systems and hardware incorporating such integratedcircuits are considered part of the embodiments described herein.

Furthermore, various layers, regions, and/or structures described abovemay be implemented in integrated circuits (chips). The resultingintegrated circuit chips can be distributed by the fabricator in rawwafer form (that is, as a single wafer that has multiple unpackagedchips), as a bare die, or in a packaged form. In the latter case, thechip is mounted in a single chip package (such as a plastic carrier,with leads that are affixed to a motherboard or other higher levelcarrier) or in a multichip package (such as a ceramic carrier that haseither or both surface interconnections or buried interconnections). Inany case, the chip is then integrated with other chips, discrete circuitelements, and/or other signal processing devices as part of either (a)an intermediate product, such as a motherboard, or (b) an end product.The end product can be any product that includes integrated circuitchips, ranging from toys and other low-end applications to advancedcomputer products having a display, a keyboard or other input device,and a central processor.

Although illustrative embodiments have been described herein withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variousother changes and modifications may be made by one skilled in the artwithout departing from the scope or spirit of the invention.

What is claimed is:
 1. A method for fabricating a semiconductorstructure, comprising: forming a first oxide layer on asilicon-germanium (SiGe) fin formed on a substrate, wherein the firstoxide layer comprises a mixture of a germanium oxide compound and asilicon oxide compound; modifying the first oxide layer to form aSi-rich outer surface of the SiGe fin; and depositing a silicon nitridelayer on the modified first oxide layer.
 2. The method of claim 1,wherein the germanium oxide compound is GeO₂.
 3. The method of claim 1,wherein the silicon oxide compound is SiO₂.
 4. The method of claim 1,wherein the first oxide layer has a thickness of about 10 Angstroms. 5.The method of claim 1, wherein the silicon nitride layer has a thicknessof about 45 Angstroms.
 6. The method of claim 1, wherein forming thefirst oxide layer comprises utilizing one or more of thermal oxidation,ozone oxidation, plasma oxidation and radical oxidation.
 7. The methodof claim 1, wherein modifying the first oxide layer comprises removingthe germanium oxide compound from the first oxide layer.
 8. The methodof claim 7, wherein removing the germanium oxide compound from the firstoxide layer comprises scavenging the germanium oxide compound from thefirst oxide layer via a first annealing process.
 9. The method of claim8, wherein the first annealing process is carried out at a temperaturefrom about 600° C. to 750° C.
 10. The method of claim 8, wherein thefirst annealing process is carried out in an atmosphere comprising atleast one of an endothermic gas and a forming gas.
 11. The method ofclaim 1, further comprising performing a shallow trench isolation (STI)process, wherein the silicon oxide layer protects the SiGe fin duringthe STI process.
 12. The method of claim 11, wherein performing the STIprocess comprises forming a second oxide layer on the silicon nitridelayer.
 13. The method of claim 12, wherein forming the second oxidelayer comprises depositing the second oxide layer via a chemical vapordeposition (CVD) process.
 14. The method of claim 13, wherein formingthe second oxide layer further comprises performing a second annealingprocess on the second oxide layer for oxide densification.
 15. Themethod of claim 12, further comprising removing a portion of the secondoxide layer corresponding to a first portion of the SiGe fin via achemical-mechanical planarization (CMP) process.
 16. The method of claim15, wherein the first portion of the SiGe fin has a first doping profileand a second portion of the SiGE fin has a second doping profile. 17.The method of claim 16, wherein the first doping profile comprises about50% SiGe.
 18. The method of claim 17, wherein the second doping profilecomprises about 25% SiGe.
 19. The method of claim 15, further comprisingrevealing the first portion of the SiGe fin
 20. The method of claim 19,wherein revealing the first portion of the SiGe fin comprises strippinga portion of the silicon nitride layer and a portion of the first oxidelayer corresponding to the first portion of the SiGe fin.